Environmentally harmful species in exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species need to be removed therefrom. In lean combustion engines, due to the existence of large amount oxygen excess, passive means without extra dosing agents, such as that using a three-way catalyst, normally are not able to effectively remove the oxidative specie NOx, as that in most of spark-ignition engines. To reduce NOx in lean combustion engines, a variety of active means with reducing agents (reductants) being dosed in exhaust gas are developed. In these technologies, a dosing amount of reductant is injected into exhaust gas, and the result mixed gas flows into a SCR catalyst, where the reductant selectively reacts with NOx generating non-poisonous species, such as nitrogen, carbon dioxide, and water.
A variety of reductants, such as ammonia (NH3), HC, and hydrogen (H2) can be used in SCR systems. Among them, ammonia SCR is used most broadly due to high conversion efficiency and wide temperature window. Ammonia can be dosed directly. However, due to safety concerns and difficulties in handling pure ammonia, in ammonia SCR systems, normally ammonia is obtained from a urea solution through thermolysis and hydrolysis, and the urea solution in these applications is also called reductant. In mobile applications, typically a eutectic solution of urea, i.e. a 32.5% wt urea solution, is used.
In a SCR system, dosing accuracy significantly affects NOx control performance, especially when engine out NOx level is high. For example, if engine out NOx is 1000 ppm, with a NSR (Normalized Stoichiometric Ratio) of 1.0, 1000 ppm ammonia is needed (in a SCR system, a NSR value is equal to a ratio of a molar amount of ammonia generated from dosed reductant to a molar amount of NOx, in an exhaust gas flow to be processed). If an uncertainty of 5% exists in dosing control, then 50 ppm of ammonia could be over- or under-generated. Though in transient, SCR catalyst has certain capability storing ammonia, damping the effects of under-dosing and over-dosing, in average, these effects may still cause issues. In the example above, if the SCR system is tuned for an average doser with zero storage usage, then an under-dosing doser may create 50 ppmNOx slip in average, which is almost 0.3 g/bhp·hr in normal operations. Compared to the US2010 emission standard of 0.2 g/bhp·hr, it is 150% uncertainty. This calculation just includes doser uncertainty. Other important factors, such as NOx sensor error, urea decomposition error, and exhaust flow rate error, also could contribute to the overall control uncertainty.
Using SCR catalyst with large storage capability (e.g. a Cu-zeolite catalyst) together with an AMOX (AMmonia OXidation) catalyst desensitizes NOx control to NSR. Thereby uncertainties in dosing system and sensors can be compensated through over-dosing. However, relying on the storage capability of SCR catalyst and AMOX could cause aging issues, since both of the storage capability of SCR catalyst and the selectivity of AMOX are subject to aging effects. An aged AMOX tends to oxidize ammonia slip back to NOx, and an aged SCR catalyst has lower deNOx efficiency. Therefore, the aging of the catalysts may cause NOx slips with over-dosing, especially when temperature variation releases stored ammonia in SCR catalyst. Additionally at high NSRs, deNOx efficiency is more sensitive to catalyst aging than at low NSRs, and the high sensitivity to catalyst aging lowers the lifetime of the catalyst.
All these issues cause difficulties in applications in which high deNOx efficiency has to be maintained. For example, to reach the requirements of 0.2 g/bhp·hr (emission limit) to 0.4 g/bhp·hr (OBD limit) set by the US2010 and CARB2016 emission regulations, when an engine out NOx level is 4.0 g/bhp·hr, a deNOx efficiency of 95% is needed for normally operations and when the deNOx efficiency drops below 90%, a fault needs to be generated. The high efficiency requirements for SCR systems cause difficulties in controlling NOx level and detecting system failures due to effects of uncertainties in dosing system and sensors, resulting in high system and warranty costs.
To lower the deNOx efficiency requirement, engine out NOx concentration has to be limited to a low level with EGR (Exhaust Gas Recirculation) technology, and retarded fuel injection. However, too much EGR and fueling retard may deteriorate engine operating performance and fuel economy. Additionally, a trade-off between NOx emission and PM emission exists in engine control. Lowering engine out NOx normally causes increase in PM emission and fuel economy is further deteriorated due to that more energy needs to be consumed in regenerating the DPF which traps PM.
In addition to the issues with high deNOx efficiency, in SCR controls, when a feedback control is used for more accurately and reliably controlling deNOx efficiency, changes in deNOx efficiency caused by catalyst aging and issues in delivering reductant require different compensations, causing difficulties in the feedback control. For example, when catalyst ages, normally deNOx efficiency cannot be increased by increasing dosing rate, and at higher NSRs, increasing dosing rate even decreases deNOx efficiency. However, if reductant solution is diluted or a dosing apparatus is under dosing, increasing dosing rate increases deNOx efficiency. The different compensation directions make the feedback control possibly go into positive feedback if the causes to changes in deNOx efficiency are unknown.
Another issue in a SCR system is temporary decrease in deNOx efficiency. Failures of a SCR catalyst, which causes low deNOx efficiency, include both permanent and temporary failures. Permanent failures, such as thermal damage, precious metal contamination, and package metal contamination, are not recoverable. Once these failures are detected for a catalyst, the catalyst then needs to be replaced. However, temporary failures, e.g., HC poison or sulfur poison caused failures, can be recovered with high temperature exhaust gas, which could be generated through post fuel injections in an engine and/or using HC dosing in a DOC/DPF system upstream from the SCR system. Temporary failures need not catalyst replacement. To lower warranty cost, permanent damages to the catalyst need to be separated from these temporary failures.
To solve the problems in a SCR system so that high deNOx efficiency can be obtained with a feedback control and more issues in the SCR system can be detected, a primary object of the present invention is to provide a multi-stage SCR control system, in which deNOx efficiency of each catalyst device is controlled according to a target value. In this system, the deNOx efficiencies can be adjusted according to different requirements. Targeting the deNOx efficiency of a front SCR device to a value with NSR lower than a stoichiometric reaction ratio decreases system sensitivity to catalyst aging and difficulties in detecting catalyst failures.
A further objective of the present invention is to provide a SCR diagnostic system, in which both of problems in catalyst and issues in dosing system and reductant quality can be detected.
Another objective of the present invention is to provide a SCR feedback control system in which a deNOx efficiency value and a reductant quality ratio value, which is indicative of reductant quality and dosing accuracy, are used in adjusting dosing control in feedback loops.
Yet another objective of the present invention is to provide a SCR diagnostic system that is able to distinguish permanent failures from temporary ones.